This fall, work began on a $91.5 million project to lessen traffic congestion on Interstate 405 near the city of Renton, in the Puget Sound area of Washington state.

Drivers commuting to employers such as The Boeing Company and Microsoft Corporation use the interstate heavily, contributing toward as much as 10 hours of congestion per day.

“The need is based on safety and operations,” said Washington State Department of Transportation project engineer Lisa Hodgson.

The project addresses the section of I-405 between Tukwila and Renton, as well as intersecting portions of State Route 167. It is entirely funded by the state, through a combination of a 5-cent gas tax increase that voters approved in 2003 and the 2005 transportation tax package.

In July, the design-build contract for the project was awarded to Bilfinger/Tri-State, a joint venture of the international company Bilfinger Berger Civil and the Pacific Northwest-based Tri-State Construction.

This is WSDOT’s fifth large-scale design-build project. So far, the method has proven successful, said Hodgson.

“I’m excited about the project, about using the mode of design-build,” she added. “It is fascinating to work in that realm.”

Currently under construction is a 1.5-mi. (2.4 km) auxiliary lane from northbound 405 to southbound SR 167. When finished, the new lane should eliminate quick merges and improve what is considered to be one of the most congested interchanges in the state.

Preliminary work to prepare for the auxiliary lane began in late September. Crews replaced the existing shoulder to put in full-depth pavement in advance of directing traffic over the shoulder.

“We’re moving into building,” said Hodgson. “We’re expanding the roadway, getting ready to build walls and to pave the area to add the auxiliary lane.”

Work on SR 167 should be completed in winter 2008.

As construction progresses on the auxiliary lane, crews will transition into the second portion of the project: adding general-purpose lanes in both directions along 2.5 mi. (4 km) of I-405, between Interstate 5 and SR 167.

Two I-405 bridges also will be improved. One bridge, which is being widened, crosses railroad tracks owned by Burlington Northern Santa Fe Railway and Union Pacific. Demolition and widening for the current bridge will have to be planned around train schedules, said Hodgson.

A second bridge will be completely replaced. Special attention will be given to seismic requirements for that structure, because the project is in a liquefaction zone.

“The bridge needs to meet the requirements for additional earthquake measures, due to soil factors that exist,” says Hodgson.

Engineers and construction crews also need to be cautious because the project is located in an environmentally-sensitive area, with wetlands and floodplains.

“The environmentally-sensitive nature of the project is probably one of the biggest challenges,” Hodgson added.

This round of improvements, which WSDOT calls “I-405 to SR 169 Stage 1 – Widening Project,” is scheduled for completion in late spring or early summer of 2010. It is part of WSDOT’s master plan for I-405 improvements.

The second stage of I-405 to SR 169 improvements will begin in 2009. The project will include a new half-diamond interchange at State Route 515, with on and off ramps to I-405; and the addition of north and southbound lanes on I-405 between SR 167 and State Route 169.

The estimated budget for improvements to I-405 from I-5 to SR 169 is $152 million. CEG

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